Photo detectors convert radiation into electrical energy. Their operation is critical to many mixed signal environments, e.g., optoelectronics, and is also useful for energy conversion, e.g., solar panels. However, typical photo detectors saturate and generate more non linear distortion at high optical powers. This limits the performance of many systems that depend upon low distortion operation. Nonlinear distortion at high optical power can be reduced by distributing photo current evenly over larger detector areas, reducing current congestion inside detectors and more uniform and better conduction of heat generated by the photo current.
Optoelectronic systems use directional couplers to transfer energy from one waveguide to another. A conventional directional coupler shown in FIG. 1A consists of two parallel waveguides that are coupled to each other via their evanescent fields within a specific interaction section where the gap between the waveguides is smallest. Energy is transferred from the input waveguide to the output waveguide.
The conventional directional coupler of FIG. 1A delivers a specific fraction of optical power from the input waveguide to the output waveguide after a specific coupling length. The percentage can range from 0% to 100%. Interaction between the input waveguide and the output waveguide in the coupling region is passive. Optical signals are passed from the input waveguide to the output waveguide. Power not transferred to the output waveguide is retained in the input waveguide. In all conventional directional couplers, the sum of the output powers in the input and output waveguides equals to the input power.
The waveguides in the conventional directional coupler of FIG. 1A are designed to have negligible propagation and coupling loss to minimize insertion loss. The two waveguide modes can be somewhat different. As long as the two waveguide modes have the same effective index, the power transfer can reach 100%. When the effective index of the modes or the coupling of the modes is changed by electro-optical effects then the power transfer is controlled electro-optically, and the device acts as a directional modulator. In the FIG. 1A device, reducing the gap of separation between waveguides in the coupling region to zero creates a multimode waveguide. When input and output waveguides are connected to a multimode section as shown in FIG. 1B, it becomes a multimode interference coupler. Conventional multimode interference couplers have uniform cross section. The interference of the modes excited by the incident input radiation distributes various fractions of power into the output waveguides after specific length L of propagation in the multimode waveguide.
Conventional photo detectors absorb incoming optical radiation and convert the absorbed photons into electrical carriers. The electrical carriers are collected by collector electrodes. The collected photo current generates an electrical signal to the external circuit. Ideally, this electric signal is directly instantaneously proportional to the variation of the absorbed optical power. In reality, the proportionality between the electrical and optical signals may be slightly nonlinear because of thermal heating, carrier screening and crowding, and other effects. These nonlinearities vary from detector to detector because of variations in their material, optical design and electrical design. For example, some photo detector designs have better thermal conduction. Some photo detectors have more uniform distribution of electrical carriers. However, the nonlinearity increases significantly at high optical power for all photo detectors.
There are two types of photo detectors: surface normal photo detector and waveguide photo detectors. In surface normal detectors, radiation is incident normal to the absorption medium which generates photo current. In waveguide detectors, radiation is incident into an optical waveguide, and it propagates down the waveguide. Absorbing media (collectors or photo detectors or detector) are imbedded into (or near) the waveguide so that the photo currents (or current) are collected together to yield the detector current.
The device capacitance C of surface normal photo detectors limits their electrical bandwidth because of the RC time constants in the electrical circuit. The device capacitance C is proportional to the detector area. Smaller detector areas therefore provide larger electrical bandwidths but also provide small signals. Smaller detector areas also have higher photocurrent density in the detector for a given incident optical power. Higher photocurrent densities can cause detector saturation. This constraint causes typical high optical power surface normal detectors to have relatively large electrode areas and low operating bandwidths.
In typical conventional waveguide photo detectors, the optical power is coupled directly to the waveguide used for detection. The optical intensity is the highest at the input end. In conventional waveguide photo detectors that use evanescent coupling between an absorbing waveguide and a passive waveguide where the optical wave is launched, the optical intensity is also highest at the input end. This still results in high peak photocurrent density in the absorbing waveguide.